Comparison of wind-stress algorithms and their influence on wind-stress curl using buoy measurements over the shelf off Bodega Bay, California

The main objectives of this study were to compare three wind-stress algorithms of varying intricacy and estimate the extent to which each method altered computed wind-stress curl. The algorithms included (1) a simple bulk formula for neutral conditions that is dependent only on wind velocity components; (2) a formula that in addition to dependence on wind components includes a simplified effect of thermal stability through differences in air and sea temperatures; and (3) an algorithm that includes full treatment of dynamics and atmospheric stability. Data for the analysis were from a field program that used a special buoy network off Bodega Bay during 28 June–4 August 2001.A diamond-shaped setup of five closely separated buoys in Bodega Bay allowed for one of the first attempts to compute wind-stress curl over the ocean using buoy measurements. Based on an analysis of the available dataset, the marine layer over Bodega Bay is characterized by positive wind-stress curl with a median value around 0.2 Pa (100 km)⁻¹ and maximum values reaching 2.5 Pa (100 km)⁻¹. Positive wind-stress curl was observed for all wind speed conditions, whereas negative wind-stress curl episodes were associated mostly with low-wind conditions.Comparison of wind-stress curl computed using the three algorithms showed that differences among them can be significant. The first and third algorithms indicated similar stress curl (difference around 10%), but the differences between these two and the second algorithm were much higher (approximately 40%). The reason for the difference is the stability correction, which in the third algorithm strongly decreases with an increase in wind speeds, but stays at a similar level for all wind speeds in the second algorithm. Consequently, for higher wind speeds the variability of wind stress calculated using the second algorithm is greater than for the other two algorithms, causing significant differences in computed wind-stress curl (root mean-square error equal to 0.19 Pa (100 km)⁻¹).Despite the apparent biases in computed wind stress and wind-stress curl among the algorithms, all of them show a significant trend of decreasing sea-surface temperature (SST) with increasing wind-stress curl. The bootstrapping analysis has revealed that both the along-shore wind stress and wind-stress curl have noticeable correlation with the changes in the sea-surface temperature as an indirect indication of the upwelling. An additional analysis, based on the low-pass filtered data, showed also significant agreement between the measured divergence in the cross-shore surface transport and the wind-stress curl computed for all three algorithms.     

Buoy measured wind, wind stress and wind stress curl over the shelf off Bodega Bay, California

An array of five buoys and three coastal stations is used to characterize the winds, stress, and curl of the wind stress over the shelf off Bodega Bay, California. The wind and wind stress are strong and persistent in the summer and weak in the winter. In the summer, wind and stress decrease strongly across the shelf, toward the coast. Combinations of buoys are used to compute the curl of the wind stress over different portions of the shelf. The mean summer 2001 curl of the wind stress over the array depends upon the area selected, varying between −1.32×10⁻⁶ and +7.80×10⁻⁶ Pa m⁻¹. The winter 2002 wind-stress curl also depends on location, varying from −2.06×10⁻⁶ to +2.78×10⁻⁶ Pa m⁻¹. Mean monthly curl of the wind stress is a maximum in the summer and a minimum near zero in the winter. In both the summer and the winter, the correlation between the wind-stress curl for different portions of the shelf varies between moderate negative, though insignificance, to high positive. A wind measurement at a single point can be poorly related to the measured curl of the wind stress at other locations over the shelf. The measurements show that the use of one wind measurement to characterize the curl of the wind stress over the shelf without further investigation of the local wind-stress curl structure is risky.     

Numerical model simulations of continental shelf flows off northern California

The three-dimensional circulation on the continental shelf off northern California in the wind events and shelf transport (WEST) experiment region during summer 2001 is studied using the primitive equation regional ocean modeling system (ROMS). The simulations are performed with realistic topography and initial stratification in a limited-area domain with a high-resolution grid. Forcing consists of measured wind-stress and heat flux values obtained from a WEST surface buoy. The general response shows a southward coastal upwelling jet of up to and a weakening or reversal of currents inshore of the jet when upwelling winds relax. Model results are compared to WEST moored velocity and temperature measurements at five locations, to CODAR surface current observations between Pt. Reyes and Bodega Bay, and to hydrographic measurements along shipboard survey lines. The model performs reasonably well, with the highest depth-averaged velocity correlation (0.81) at the inshore mooring (40 m water depth) and lowest correlation (0.68) at the mid-depth mooring (90 m depth). The model shows generally stronger velocities than those observed, especially at the inshore moorings, and a lack in complete reversal of southward velocities observed when upwelling winds relax. The comparison of surface velocities with CODAR measurements shows good agreement of the mean and the dominant mode of variability. The hydrography compares closely at the southern and northern edges of the survey region (correlation coefficients between 0.90 and 0.97), with weaker correlations at the three interior survey lines (correlation coefficients between 0.44 and 0.76). Mean model fields over the summer upwelling period show slight coastal jet separation off Pt. Arena and significant separation off Pt. Reyes. The cape regions also experience relatively strong bottom velocities and nonlinearity in the surface flow. Across-shelf velocity sections examined along the shelf reveal a double jet structure that appears just north of Bodega Bay and shows the offshore jet strengthening to the south. We examine the dynamics during an upwelling and subsequent relaxation event in May 2001 in which the WEST measurements show evidence of a strong flow response. The alongshelf variability in the upwelling and relaxation response introduced by Pt. Reyes is evident. Analysis of term balances from the depth-averaged momentum equations helps to clarify the event dynamics in different regions over the shelf. A clear pattern in the nonlinear advection term is due to the spatial acceleration of the southward jet around the capes of Pt. Arena and Pt. Reyes during upwelling. Results from a three-dimensional Lagrangian analysis of water parcel displacement show significant southward displacement in the coastal jet region, including a strong signal from the double jet. Alongshelf variability in parcel displacements and upwelling source waters due to the presence of Pt. Arena and Pt. Reyes is also apparent from the Lagrangian fields. A cyclonic eddy-like recirculation feature offshore of Pt. Arena prior to the upwelling event causes large patches of onshore-displaced parcels. Additionally, across-shelf variability in the response of water parcels along the D line includes decreased vertical displacement and increased alongshelf displacement in the offshore direction.     

Harmonic analysis of tides and tidal currents in South San Francisco Bay,California

Water level observations from tide stations and current observations from current-meter moorings in South San Francisco Bay (South Bay), California have been harmonically analysed. At each tide station, 13 harmonic constituents have been computed by a least-squares regression without inference. Tides in South Bay are typically mixed; there is a phase lag of approximately 1 h and an amplification of 1·5 from north to south for a mean semi-diurnal tide. Because most of the current-meter records are between 14 and 29 days, only the five most important harmonics have been solved for east-west and north-south velocity components. The eccentricity of tidal-current ellipse is generally very small, which indicates that the tidal current in South Bay is strongly bidirectional. The analyses further show that the principal direction and the magnitude of tidal current are well correlated with the basin bathymetry. Patterns of Eulerian residual circulation deduced from the current-meter data show an anticlockwise gyre to the west and a clockwise gyre to the east of the main channel in the summer months due to the prevailing westerly wind. Opposite trends have been observed during winter when the wind was variable.     

Nearshore retention of upwelled waters north and south of Point Reyes (northern California)—Patterns of surface temperature and chlorophyll observed in CoOP WEST

Retentive embayments can be found near capes in upwelling regions, where they stand out as relatively warm features with higher chlorophyll a concentrations than surrounding waters. Within the area of the “Wind Events and Shelf Transport” study site (WEST) from the Gulf of the Farallones to Point Arena (37.5–39°N and 122.5–124°W), we describe two retentive embayments, extending approximately 20 km north of Point Reyes, and 30 km south of Point Reyes in the northern Gulf of the Farallones. These areas are identifiable from chlorophyll a (SeaWiFS) and sea-surface temperature (MODIS) satellite radiometry. Additional data from moorings and drifters were used to further characterize these retentive features. The persistence of these features is on time scales between 2 and 10 days, determined from time-series analysis of mooring data for chlorophyll and temperature. The alongshore wind stress is negatively correlated with chlorophyll and temperature mooring values. The time scale of persistence of these retentive features was similar to both the upwelling-relaxation timescale and the timescale of phytoplankton bloom development. The WEST region is notable in that the spring and summer is subject to the strongest wind stress along the US west coast, yet the coastal waters are highly productive. Based on our observations, we suggest that the apparent persistence of high biomass coastal waters in this strongly advective and wind-dominated system may be partially explained by the presence of retentive features identified in this study.     

Observations and measurements of wave-induced drift of surface inextensible film in deep and shallow waters

The present study investigates the drift of two-dimensional floating surface films by both deep and shallow water waves in the laboratory. The focus is on the observations and measurements on the drift behavior and to clarify some outstanding issues. Thin polyethylene sheets with reflective markers were attached to simulate the inextensible surface films. Upon the initiation of a wave train, two infrared cameras recorded continuously the instantaneous position of the markers at 50 Hz. The temporal variation of the drift velocity was then determined by processing the recorded data. In all experiments, the drift velocity increased quickly in the beginning and reached a quasi-steady mean value. The magnitude of the surface drift typically increased with the longitudinal length of the polyethylene sheet until the sheet length was approximately the same as the wavelength. Lengthening the sheet length further did not lead to significant changes in the drift velocity. Under the shallow water condition, the wave-induced drift velocity increased linearly with the Ursell number and with a lower water depth.     

大亚湾表层沉积物镁铝含量比变化及影响因素

本文讨论了大亚湾沉积物镁铝含量比——m值的分布、季节变化及m值与水介质的关系,结果证实,水介质的盐度、pH和水温是m值的主要影响因素,且水介质的盐度是决定m值大小的最基本要素。当3个影响因素呈同向变化时,对m值的变化具有叠加作用,m值的变化幅度较大;当盐度与pH呈相反方向变化时,若△pH>0.06,可对m值产生明显的影响,m值的变化与pH同向。从大亚湾沉积物m值随季节变化的特征表明,m值对沉积环境中水介质变化的反应是灵敏的。这一结果显示了m值对水介质变化能够响应的灵敏度。     

Mercury methylation,demethylation and reduction rates in coastal and marine surface waters of the Mediterranean Sea

In situ experiments using isotopically labeled mercury species (¹⁹⁹Hg(II) and Me²⁰¹Hg) are used to investigate mercury transformation mechanisms, such as methylation, demethylation and reduction, in coastal and marine surface waters of the Mediterranean Sea. The aim of this work is to assess the relative contribution of photochemical versus biological processes to Hg transformation mechanisms. For this purpose, potential transformation rates measured under diurnal and dark incubation conditions are compared with major biogeochemical parameters (i.e. hydrological and biological data) in order to obtain the relative contribution of various biotic and abiotic mechanisms in both surface (high light) and bottom (low light) waters of the euphotic zone. The results demonstrate that coastal and marine euphotic zones are significant reactors for all Hg transformations investigated (i.e. methylation, demethylation, reduction). A major outcome demonstrates that Hg methylation is taking place in oxic surface seawater (0.3–6.3% day^− 1) and is mainly influenced by pelagic microorganism abundance and activities (phyto- and bacterioplankton). This evidences a new potential MeHg source in the marine water column, especially in oligotrophic deep-sea basins in which biogeochemistry is mostly governed by heterotrophic activity. For coastal and marine surface waters, although MeHg is mainly photochemically degraded (6.4–24.5% day^− 1), demethylation yields observed under dark condition may be attributed to microbial or chemical pathways (2.8–10.9% day^− 1). Photoreduction and photochemical reactions are the major mechanisms involved in DGM production for surface waters (3.2–16.9% day^− 1) but bacterial or phytoplanktonic reduction of Hg(II) cannot be excluded deeper in the euphotic zone (2.2–12.3% day^− 1). At the bottom of the euphotic zone, photochemical processes are thus avoided due to the attenuation of UV-visible sunlight radiation allowing biotic processes to be the most significant. These results suggest a new potential route for Hg species cycling in surface seawater and especially at the maximum biomass depth located at the bottom of the euphotic zone (i.e. maximum chlorophyll fluorescence). In this environment, DGM production and demethylation mechanisms are thus probably reduced whereas Hg methylation is enhanced by autotrophic and heterotrophic processes. Experimental results on mercury species uptake during these investigations further evidenced the strong affinity of MeHg for biogenic particles (i.e. microorganisms) that correspond to the first trophic level of the pelagic food web.     

1986年秋日本以南和以东海域仔稚鱼的分布

１９８６年１０～１１月在日本以南和以东海域的４１个测站进行了鱼卵、仔鱼的调查，共采集到９９６尾仔稚鱼，它们隶属于３８科６７种，经济鱼类仔鱼稀少，而大洋性发光鱼类仔鱼十分丰富，其中钻光鱼科和灯笼鱼科分别占仔鱼总数的５０．５０％和３２．３３％。优势种为细钻光鱼、串灯鱼和西明灯鱼，它们分别占仔鱼总数的２５．３０％，１６．４７％和７．５３％。仔鱼的丰度在黑潮流轴外侧边缘及其邻近的大洋域最高，在日本以南的沿岸水域占第二位，而在日本以东的混合水域最低。仔鱼丰度与温、盐度之间有显著的正相关关系。     

Spatial distribution and corresponding determining factors of metal concentrations in surface sediments of Beppu Bay, southwest Japan

This study determined the factors contributing to the spatial distribution of 14 metal concentrations in the surface sediments of Beppu Bay on the basis of comparisons of the organic geochemical properties and environmental parameters through principal component analysis (PCA) and redundancy analysis (RDA). The results of PCA and RDA showed that the concentrations of V, Cr, Co, and As were closely related to the distances between the sampling sites and the Oita River. This indicated that these metals originated from the river's drainage area. The Mn, Cu, Mo, and Cd concentrations were related to the water depth. These results indicated that the Mo, Cd, and Cu deposition processes were controlled by oxygen depletion, and that these elements accumulated in the deeper parts of the bay under anoxic conditions.     

The composition of phytoplankton within the Chesapeake Bay plume and adjacent waters off the Virginia coast,U.S.A.

Phytoplankton composition differences were used to identify and plot the extent of the Chesapeake Bay plume over the continental shelf. Distinct but different phytoplankton assemblages were noted within the plume for March, June and October 1980. The major constituents in the plume were Skeletonema costatum, a variety of other diatoms and an unidentified ultraplankton component assumed to consist of several cyanophycean and chlorophycean species. The shelf plankton outside of the plume was mainly characterized by the coccolithophores and other phytoplankters that formed different seasonal assemblages than what was within the plume. Patchiness and fluctuations in the shape, size and extent of the plume were noted. A total of 235 species is given, with their presence within the plume and at shelf stations indicated.     

Nutrients,light and phytoplankton production in the shallow,tropical coastal waters of Bandon Bay,Southern Thailand

Phytoplankton primary production and its regulation by light and nutrient availability were investigated in the shallow, tropical coastal waters of Bandon Bay, Southern Thailand. The bay was meso‐eutrophicated and highly turbid, receiving river water discharge. Water column stratification was consistently weak during both rainy and dry seasons. Dissolved inorganic nitrogen (DIN) was higher off the river mouth than in the other regions, suggesting that river water discharge was a main source of DIN. By contrast, dissolved inorganic phosphorus (DIP) showed a significant negative correlation with total water depth, implying that regeneration around the sea floor was an important source of DIP. Surface DIN and DIP showed positive correlations with surface primary production (PP) and water column primary productivity (ΣPP*), respectively. The combined correlation and model analyses indicate that total water depth had an ambivalent influence on water column primary production (ΣPP); shallower water depth induced more active regeneration of nutrients, but it also caused higher turbidity and lower light availability as a result of enhanced resuspension of sediments. Furthermore, there was a vertical constraint for phytoplankton during the rainy season: total water depth tended to be shallower than euphotic zone depth. In conclusion, light limitation and vertical constraint owing to shallow water depth appear to be more important than nutrient limitation for water column primary production in Bandon Bay.     

东山乌礁湾海滩表层沉积物粒度时空变化分析

为了进一步了解海岛岬湾海岸表层沉积物粒度的时空分布特征和形成机制,作者以福建东山岛乌礁湾为典型研究区,通过秋、春、夏3个不同季节海滩表层沉积物的采集和点位测量,综合应用激光法和筛析法进行粒度测定。结果表明,东山乌礁湾海滩表层沉积物以0.16～0.50 mm之间的中、细砂为主,并含有少量的粗砂和细砾,这与区域砂质沉积背景有关;湾内从南到北剖面沉积物粒径由粗砂到细砂逐渐变细,主要受剖面地形、局地物源、季节性风浪作用和近岸往复水动力的影响。时间变化上,各取样站位表现出沉积物粒径粗、细不同程度的多种变化趋势,以秋季为参考,整体上表现为由南到北粒级的变小、增大和稳定,这与东北、南南西季风影响下的浪、潮作用以及沿岸流系格局变化下的水动力环境有关。     

2020年冬季大亚湾西南海域主要渔业生物碳氮稳定同位素研究

为了解大亚湾西南海域食物网的营养结构特征, 本研究于2020年1月份使用底拖网采集了该海域的渔业生物, 并分析了35种主要渔业生物的碳氮稳定同位素值。根据δ¹³C和δ¹⁵N值, 计算出该海域食物网6种营养结构的生态指标和主要渔业生物的营养级, 并绘制了连续营养谱。本次调查渔业生物主要为鱼类和虾蟹类, 鱼类的δ¹³C和δ¹⁵N值范围分别为-17.63‰ ~ -14.85‰和12.92‰~15.46‰, 平均值分别为-16.47‰和13.80‰; 虾蟹类的δ¹³C和δ¹⁵N值范围分别为-17.67‰ ~ -15.51‰和11.05‰~12.62‰, 平均值分别为-16.30‰和11.85‰。根据δ¹⁵N值, 用相加模型(trophic position by the additive model, TPA)和缩比模型(trophic position by the scaled model, TPS)分别计算了主要渔业生物的营养级, 结果显示两个模型计算的结果无显著性差异(P>0.1), 呈现鱼类平均营养级>虾蟹类的趋势。本研究发现大亚湾西南海域食物网初始食物来源较为单一, 存在食物链营养层级较少和长度不足, 食物网营养级多样性较低和营养结构冗余程度高的现象。与30多年前相比, 大亚湾近年高营养级生物量减少, 食物网结构由复杂趋向简单化, 生态系统稳定性较差。本研究结果不仅为了解大亚湾食物网结构组成提供了基础资料, 也为保护大亚湾渔业资源, 维持生态系统结构的稳定性提供参考依据。